Low hydrogen embrittlement  zinc/nickel plating for high strength steels

ABSTRACT

The invention provides a method for producing a corrosion-resistant article, where the article is conductive and subject to hydrogen uptake during electroplating of a coating. The method comprises electroplating a zinc/nickel coating on the article in an aqueous, basic plating solution containing zinc and nickel ions. The method uses an electrolyte in the form of a soluble hydroxide salt with the weight ratio of zinc ions to nickel ions in the solution being sufficient to provide the coating comprising from about 85% to about 95% by weight zinc, and about 5% to about 15% by weight nickel. The plating solution is substantially free of brightening agents which retard hydrogen bake-out.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/126,645 filed on May 11, 2005. The disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present invention relates to electroplating an article with azinc/nickel alloy and to zinc/nickel electroplated articles made of lowalloy high strength steel exhibiting corrosion resistance and lowhydrogen embrittlement.

BACKGROUND

High strength steels are employed on commercial aircraft components suchas landing gears, flap tracks and other high load items. Conventionally,special purpose coatings such as chrome deposits for wear or cadmiumdeposits for corrosion protection are deposited electrolytically fromaqueous electroplating baths. Electroplating tends to liberate hydrogenfrom the cathode, which can lead to hydrogen embrittlement unlessremoved through controlled baking after plating.

High-strength low alloy steels are sensitive to delayed, brittlefailures at certain low stresses. Such failures have been attributed tothe presence of hydrogen in the steel microstructure. The hydrogen canbe introduced into the microstructure by reaction with water or aqueoussolution, or, by electrochemically discharging hydrogen at the surfaceof the steel. Since high-strength steels have corrosion-resistantcoatings that are applied by electroplating techniques, hydrogen isdischarged onto the steel surface along with the corrosion-resistantcoating. Therefore, the quantity of hydrogen deposited at the coatingsteel interface must be carefully monitored and controlled.

In order to bake out hydrogen from these steels, a plate deposit ofspecific structure and morphology is required in order to allow thehydrogen to pass through the plating. For this purpose the aircraftindustry and others have used low embrittlement processes such asBAC5709, “Hard Chromium Plating” and BAC5804, “Low HydrogenEmbrittlement Cadmium-Titanium Alloy Plating”.

Cadmium-titanium electroplating of high strength low alloy steelsprovides suitable resistance to hydrogen embrittlement. A Cd—Ti alloy iselectroplated onto high-strength steels under carefully controlledconditions. The resulting plated product is then heat treated atelevated temperatures to achieve an acceptable low hydrogenembrittlement level. It is believed that the porosity of theelectroplated cadmium-titanium alloy is the key to the removal of thehydrogen during a subsequent heat treatment operation. It must be notedthat the cadmium-titanium plating bath is sensitive to contamination, socare must be taken to achieve acceptable embrittlement characteristics.And toxic components in the bath lead to health and environmentalproblems. For example, the cadmium-titanium alloy plating bath containscadmium and cyanide, which create disposal problems unless expensivewaste treatment equipment is employed.

Zinc/nickel alloys have been suggested for electroplating onto steels torender them corrosion-resistant. Such zinc/nickel baths are free ofcadmium and cyanide free and contain relatively non-toxic components.Some have provided acceptable corrosion protection, but most havedisadvantages such as difficulty in controlling the process, andunacceptably short bath life.

There continues to be a need for an improved coated product havingcorrosion resistance, low hydrogen embrittlement and which is non-toxic,especially as it relates to high strength low alloy steels for theaerospace industry.

SUMMARY

In summary, in the past, there was an attempt to achieve corrosionprotection of high strength steels while avoiding hydrogen embrittlementthrough cadmium plating. Cadmium is a known carcinogen and airpollutant. The process also uses cyanide and special precautions areneeded to avoid releasing this as a harmful gas. An acid zinc/nickelplating process that is used for lower strength steels poses problemswith hydrogen embrittlement, particularly for higher strength steels.

Drawbacks of the prior process are avoided and advantages obtainedthrough the practice of a method for producing a corrosion-resistantarticle. The article is especially selected from high strength low alloysteels such as are used for high strength parts in the aerospaceindustry. The method comprises electroplating a zinc/nickel coating onthe article from an alkaline plating bath. The method uses anelectrolyte in the form of a soluble hydroxide salt with the weightratio of zinc ions to nickel ions in the solution being sufficient toprovide the coating comprising from about 85% to about 95% by weightzinc, and about 5% to about 15% by weight nickel. In an innovation, theplating solution is maintained substantially free of organic materialsthat interfere with formation of a plating resistant both to corrosionand to hydrogen embrittlement. It has been discovered that suchinterfering materials include many that serve as conventional“brighteners” for plating of other alloys.

In certain embodiments, the plating solution contains no organicmaterials at all. In other embodiments, the plating solution contains anorganic material that acts as a nickel complexing agent withoutinterfering with plating a resistant coating.

By using the plating baths, for the first time so-called “dense, porous”plating or coating can be applied on high strength low alloy steels thatprovide resistance both to general corrosion and to hydrogenembrittlement. Corrosion protection is provided by the dense nature ofthe coating. Favorable hydrogen embrittlement properties are provided bythe porous nature of the coating. The release of previously-absorbedhydrogen is achieved by various methods whereby hydrogen diffuses withinthe material and is outgassed from the material. The release of hydrogenvia bake-out occurs typically at elevated temperature over a period oftime.

Thus, in various embodiments, plated articles are provided that have ahigh strength low alloy steel substrate and a Zn/Ni plated coating in adense, porous morphology that provides an desirable combination ofcorrosion resistance and low hydrogen embrittlement.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a process flow diagram;

FIG. 2 a is a low embrittlement alkaline Zn—Ni plate deposit of theinvention at 500×, top surface.

FIG. 2 b shows a bright Zn—Ni plate deposit (hydrogen bake-outinhibited) at 500×, top surface.

FIG. 3 a shows the plate of FIG. 2 a at a plate bend fracture at 500×.

FIG. 3 b shows the plate of FIG. 2 b in a plate bend fracture at 2000×.

FIG. 4 a shows a top view of standard low embrittlement Cadmium-Titaniumplate at 500×, showing a coating on a HSLA steel;

FIG. 4 b shows a standard dense bright cadmium deposit at 500×;

FIGS. 5 a and 5 b show the platings of 4 a and 4 b, respectively, in aplate bend fracture;

FIG. 6 a shows a coating on an aluminum-nickel bronze steel rod; and

FIG. 6 b shows a coating on 4340M steel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

In one embodiment, a method is provided for producing a plated low alloyhigh strength steel that has superior corrosion and low hydrogenembrittlement properties. The method involves electroplating azinc/nickel coating on a low alloy high strength steel substrate from analkaline plating bath. The plating bath contains zinc and nickel at alevel that will produce a plated coating containing 85-95% by weightzinc and 5-15% by weight nickel. Preferably, the bath contains aneffective amount of a nickel complexing agent. In a preferredembodiment, the bath contains essentially no components that are abrightener for zinc, nickel, electroplated coatings. In particular, thebath preferably contains less than 100 ppm, and preferably less than 10ppm or less than 1 ppm of any organic material other than the complexingagent. Advantageously, the coating applied by such a method has a dense,porous morphology that provides corrosion resistance on account of itsdensity and resistance to hydrogen embrittlement on account of itsporosity.

In various embodiments, the alkaline electroplating bath has a pH of12-13.5 and contains the zinc, nickel, and complexing agent describedabove. Generally, it is preferred to control the process by usingsuitable levels of hydroxide as discussed herein. Depending on a numberof factors, including the buffering capacity of the bath a pH of 12-13.5is achieved.

In one aspect, the invention is based on the discovery that a highperformance zinc nickel coating can be plated by using optimizedparameters that, because of the lack of any organic material other thanthe complexing agent, lead to coatings on low alloy high strength steelsthat have a combination of corrosion and hydrogen embrittlementproperties. Thus in various embodiments, the hydroxide concentration ofa bath is held 17.4-21.2 ounces per gallon (normally resulting in a pHof 12-13.5) and the cathode current density during electroplating is30-60 amperes per square foot. Other preferred parameters for theelectroplating include a bath temperature of 70° F.-80° F., a content ofzinc metal ranging from 1.17-1.45 ounces per gallon, a composition ofnickel metal in the bath ranging from 0.12-0.15 ounces per gallon, andthe weight ratio of zinc to nickel in the bath ranging from 10:1 to11:1. In a preferred embodiment, all of the bath and coating parametersmentioned above are combined. It has been determined that, by leavingout conventional brightening agents, coatings of suitable morphology andcorrosion and embrittlement characteristics can be produced on highstrength low alloy steels.

In another embodiment, the invention provides a method or protocol forpreparing a low alloy high strength part for electroplating, followed byelectroplating and hydrogen embrittlement relief baking theelectroplated part. Thus, in one embodiment, the invention provides amethod comprising an aqueous degreasing of the steel part, followed bygrit blasting the degreased part. Thereafter, the grit blasted steelpart is activated before electroplating it in an alkaline Zn/Ni platingbath. After electroplating, the part is subjected to a relief baking toremove hydrogen and improve hydrogen embrittlement. Optionally andpreferably, a chromate conversion coat is applied onto the relief bakedpart. Electroplating is carried out under the conditions and parametersdescribed herein.

An advantage of the methods is that it provides zinc/nickel plated steelparts that combine corrosion resistance and low hydrogen embrittlement.In various embodiments, the useful combination of properties isascribable to a dense and porous morphology of the plating that forms onthe part during the electroplating step and subsequent bake out.

Thus, in one embodiment, the invention provides a zinc nickel platedarticle made of a substrate and a zinc/nickel alloy plating on thesubstrate. In particular embodiments, the substrate is a high strengthlow alloy steel and the plating contains 85-95% by weight zinc and 5-15%weight nickel. As noted, the morphology of the plating is dense andporous, such that the article meets corrosion specification requirementswhen tested in accordance with ASTM B-117 and the low embrittlementspecification requirements when tested in accordance with ASTM F-519,Type 1a.2. In preferred embodiments, the substrate is a steel alloy suchas AISI 4340M, and/or the article is part of a commercial air craftcomponent. The plated article is made, in preferred embodiments, by anyof the methods described herein. As noted, the plating on the substrateis characterized by a morphology sufficiently dense to provide theobserved corrosion resistance and adequately to provide egress channelsto meet the porous low hydrogen embrittlement characteristics. That is,the plating has a density sufficient to provide corrosion resistance anda porosity sufficient to provide hydrogen embrittlement resistance.

In various embodiments, a zinc/nickel alloy is electrodeposited onto avariety of electrically conductive substrates in accordance with thepresent invention. The zinc/nickel alloy deposited in accordance withthe present invention is especially efficacious, however, when appliedto high-strength steel articles that are sensitive to hydrogenembrittlement. Not only is the steel article rendered corrosionresistant, it also exhibits little or no hydrogen embrittlement afterthe electroplating process. Accordingly, the present invention providesa viable and effective substitute for prior cadmium-titaniumcorrosion-resistant coatings.

High-strength steels are generally those steels having a tensilestrength, as a result of alloys being heat-treated, of about 200 ksi orhigher, and more particularly 220 ksi or higher. High strength low alloysteels (HSLA steel) is a term given to high strength steels that have arelatively low level of alloying, such as below 20% or at 10% or below.Typical alloying elements in HSLA steels include Cr and Ni. In oneaspect, HSLA steels are distinguished from so-called stainless steelswhich, because of their relatively high content of alloying metals (forexample Cr) are designated high alloy steels.

The alloy steels coated as described herein exhibit a favorablecombination of corrosion resistance and hydrogen embrittlementresistance. In various embodiments, the plated high strength low alloysteels meet corrosion resistance requirements of ASTM B-117 and the lowhydrogen embrittlement requirements of ASTM F-519, Type 1a.2

The invention is particularly advantageous to high strength low alloysteels having the stated tensile strength or higher. However, theinvention is also useful for other steels and metals, such as copper,which are susceptible to hydrogen uptake during electro-deposition in asolution that liberates hydrogen. The invention is also useful inplating low strength steels such as AISI 4130, carbon steels, andstainless steels. The invention is also useful for coating otherconductive substrates such as graphite. The invention is an alternativeto acid zinc/nickel bath plating, and other plating baths containingagents that facilitate hydrogen embrittlement.

The plating bath formed in accordance with the present invention is anaqueous solution containing zinc, nickel, and an electrolyte in the formof a soluble hydroxide salt, preferably along with a nickel complexingagent. The metal cations are placed in an aqueous solution of asolubilized oxide or salt of the cation. It is preferred, as will bediscussed in more detail below, that the solution contains hydroxideions. Brighteners are not included in the solution.

The zinc cations can be provided by a variety of water-soluble zinccompounds. The water-soluble compounds include zinc hydroxide and otherzinc salts such as zinc sulfate, zinc oxide and, of course, the variouscombinations and mixtures thereof. Zinc content is achieved and alsoreplenished by a variety of means. Concentration of the zinc salt shouldbe sufficient to provide at least about one ounce of zinc ion per gallonof solution. Preferably, the zinc ions should be present in an amountranging from about 1.1 to about 1.5 ounces per gallon of solution, andmore preferably from about 1.17 to about 1.45 ounces per gallon. Theconcentrations are based on the weight of zinc. An appropriate amount ofzinc salt is added to the bath to provide those concentration levels.

The nickel cations can be provided from a variety of water-solublenickel salts, including nickel sulfate, nickel fluoroborate, nickelacetate, and the various mixtures and combinations thereof. Nickelsulfate is preferred. The nickel salt should be present in an amountsufficient to provide at least about 0.1 ounces of nickel ion per gallonof solution. It is preferably about 0.1 to about 0.2 of nickel ion, andmore preferably about 0.12 to about 0.15, ounces per gallon. As withzinc, the concentration levels are based on the weight of nickel ion.Stoichiometric considerations determine how much nickel salt to add tothe bath to provide these concentration levels.

In particular embodiments, and particularly when a preferred highstrength low alloy steel is to be plated, it is preferred that theweight ratio of zinc ion to nickel ion in the solution be in the rangeof from 10:1 to 11:1. Such levels have been shown to provide a platedcoating exhibiting advantageous combinations of corrosion resistance,low hydrogen embrittlement, and re-embrittlement characteristics.

The conductivity of the electroplating bath is increased by the presenceof the electrolyte. The preferred electrolytes include soluble hydroxidesalts of metals, preferably Group I alkali metals and most preferably,sodium hydroxide. Various mixtures and combinations of metal hydroxidesmay be used. The sodium hydroxide should be present in the bath in anamount sufficient to provide from about 15 to about 25 ounces of sodiumhydroxide per gallon of solution, preferably from about 17.4 to about21.2 ounces per gallon of solution. Naturally, the electrolytes providedby the metal hydroxides also increase the pH of the bath. If necessary,the pH can be adjusted with an acid such as sulfuric acid to achieve adesired value. In preferred embodiments, the pH of the electroplatingbath is adjusted to a basic range, preferably pH greater than 12 andless than 14. An exemplary pH range is from about 12 to about 13.5.

Although the bath can contain agents to help keep the nickel ions insolutions, conventional brightening agents, such as organic brighteners,are not included in the solution. Conventional brighteners, andparticularly organic brighteners are employed in the art to providebright, specular deposits. However, in preferred embodiments the presentinvention avoids brighteners, and indeed any organic material except forthe nickel complexing agents. By the present invention, it has beenobserved that the use of such brighteners in the electroplating bathtends to lead to plating deposits that are sufficiently dense to obtaingood corrosion resistance, but insufficiently porous to providefavorable embrittlement properties, especially in the case of platedHSLA alloys. It is believed that the dense platings retard or otherwisehinder hydrogen bake-out, a situation that is surprisingly avoided byomitting the brighteners from the bath.

Conventional brighteners are materials that alter the structure of theplate deposit with respect to plate morphology through grain refinement.Their use reduces porosity of the plating and increases platingsmoothness. Although such effects are desirable in conventional steels,it has now been determined that such is undesirable for HSLA steels. Theinvention is not limited by theory, but it appears that the noted grainrefinement can lead to reduced hydrogen permeability, which in HSLAsteels may lead to base material embrittlement.

Production of zinc/nickel coatings on high strength low alloy steelshaving suitable morphology and corrosion/embrittlement propertiesdepends on the bath chemistry as described above and also on the stepsused in preparation of the alloy for coating and the parameters of theelectrodeposition process.

The zinc/nickel plating process is carried out in the pH, temperature,and current density ranges, suitable as described herein. When carriedout with a preferred solution as outlined above and within the preferredoperating ranges set forth herein, an alloy containing from 85 to 95percent by weight zinc and 5 to 15 percent nickel is produced. Whenapplied to HSLA alloys, this coating will provide excellent corrosionresistance as well as low hydrogen embrittlement. In a preferredembodiment, the coating consists of about 90 weight percent zinc andabout 10 weight percent nickel. The goal is to combine maximum corrosionresistance and minimum hydrogen embrittlement.

The bath can effectively be operated in a basic pH range, preferablyfrom about 12 to about 13.5. In one embodiment, the pH of the bath isadjusted as need be, while avoiding the introduction of undesirable ionsinto the solution. In other embodiments, the amount of hydroxide ion iscontrolled resulting in a basic pH throughout the plating process. Thezinc/nickel alloy can be plated in accordance with the present inventionover a wide variety of temperatures and current densities. Theelectrodeposition can occur over a broad temperature range of about 65°F. to about 85° F., preferably at room temperature from 70° F. to 80° F.

The cathode current densities preferably range from about 30 to about 68amperes per square foot (ASF) or from 30 to 60 ASF to yield asatisfactory corrosion-resistant coating and also achieve low hydrogenembrittlement on HSLA steels. In general, these current duties arehigher than those used in conventional bright plating, which aretypically in the range of 10-30 ASF. In one aspect, avoidance of organicbrighteners in the plating bath surprisingly permits the process to becarried out in a current density range not rechargeable withconventional plating.

At a current density that is too low on the order of 20 to 24 ASF, thereis a tendency toward poor coverage of the substrate with the coating. Athigh current density, on the order of over 70 amperes per square foot,for example, the metal is deposited more rapidly and the coating tendsto appear rough or relatively grainy, yielding unsatisfactory results.

The anode of the electroplating system is preferably nickel. The partbeing treated is the cathode. The resulting product has nickeldistributed throughout the zinc coating. At the typical 36 amperes persquare foot the deposition preferably occurs over a period of time of 15to 30 minutes of plating depending on the thickness requirement. The 30minute plating rate (PR) for this process is approximately 0.8 mil(thousand of an inch) at 30 ampere-per-square-foot (ASF), 1.5 mils at 45ASF and 1.9 mils at 68 ASF, although plating rate can significantly varydepending on combined operating condition and parameters.

The zinc/nickel coatings of the invention are preferably applied to athickness of 0.0005 to 0.0008 inch (0.5 to 0.8 mil) for aerospaceapplications. Plating time to achieve such coatings is dependent on thecurrent density, temperature, and the composition of the bath

The complexing agent (chelating agent) plays a role in giving sufficientnickel ion concentration and distribution, even at a low concentration.Thus, the complexing agent facilitates distributing the nickelco-precipitation with zinc in a uniform ratio. A complexing agent istherefore preferably included in the electroplating bath at a level toprovide suitable complexing and bath stability. Suitable complexingagents include those that effectively complex nickel and that do notundergo reaction such as electrolysis under the plating conditions. Itis believed that such lack of reaction contributes to the stability ofthe complexing agents and also to the lack of participation by thecomplexing agent in side reactions that would be deleterious to theformation of a coating with the desired dense and porous morphology thatleads to the desirable combination of corrosion resistance and lowhydrogen embrittlement performance.

If needed, suitable complexing agents can be identified by routineexperimentation to determine whether a plating bath containing thecomplexing agents produces a coating on HSLA steel that provides bothcorrosion and embrittlement properties. If a proposed complexing agentinterferes with such plating or coating, another can be used in itsplace. It has thus been discovered that suitable coating are produced onHSLA steels by careful avoidance of additives in the plating bath thatare otherwise conventional in the zinc/nickel plating of other steelalloys.

One exemplary class of complexing agents is an organic amino compound,such as ethylenediamine (EDA), polyethylenepolyamines such as diethylenetriamine (DETA), triethylenetetramine (TETA), N-hydroxyethylenediamine(HEEDA), etc., and alkanol amines such as diethanolamine (DEA),triethanolamine (TEA), N-methylethanolamine, 2-aminopropanol, and thelike. In various embodiments, DETA or a combination of DETA and TEA arepreferred. The complexing agent may include aminocarboxylic acids orsalts such as nitrilotriacetate, ethylenediamine-tetraacetate,diethylenetriamine pentaacetate and so on. It is understood that anaminocarboxylic acid complexing agent will be in a salt (deprotonated)form at the alkaline bath pH. Other complexing agents can be selected iftheir use does not produce unsuitable coatings, with regard to corrosionand hydrogen embrittlement properties.

The zinc/nickel plating baths contains less than 100 ppm of any organicmaterial other than one that meets the requirements of a nickelcomplexing agent described herein. Preferably, there is less than 50 ppmof such organic material, or less than 10 ppm. In a preferredembodiment, the plating bath compositions contain essentially no organicmaterial other than chemical species that complex and keep nickel ionsin solution and do not undergo electrolysis reaction under the platingconditions. In various aspects, this means that conventional brighteningagents are to a large extent avoided. A brightening agent is one that,if included, would lead to a coating characterized as “bright” by thoseof skill in the art. Bright coatings are desirable in some applications;however, in the current invention, the presence of brighteners and theresulting bright coatings on high strength low alloy steels has beenfound correlated with unacceptable corrosion and/or hydrogenembrittlement properties. Normally, the resulting zinc/nickel coatingsof the invention are found to be matte in appearance, while possessinghigh corrosion resistance and low to hydrogen embrittlement.

Agitation of the bath during plating is preferably carried outmechanically. In various aspects, air agitation (e.g. bubbling andsparging) is avoided because of the sensitivity of the process tooxygen.

Plating is carried out preferably with an anode to cathode ratio of atleast 2:1. With conventional bright coating lower ratios can be used.With the current method, larger anodes are used to provide good hydrogenembrittlement properties. Also, with the high current densities used inthe present invention auxiliary anoding is important and is preferablyused. Suitable auxiliary anodes include nickel, nickel plates, andplatinized titanium anodes.

Prior to deposition of the zinc/nickel alloy, the article to be platedis preferably cleaned and activated for electrodeposition. The articleto be plated is first manually solvent cleaned or vapor degreased inaccordance with conventional procedures normally employed in the platingarts. After cleaning or degreasing, the article is dried and cleanedwith an abrasive blast. Within a relatively short period of time afterthe abrasive cleaning, the article is rinsed in cold water for severalminutes, activated by immersing it in an acid or other solution for adesired time, rinsed with cold water, transferred, immersed and platedelectrolytically in the zinc/nickel plating bath as described herein.

A chromate treatment of the part can follow the hydrogen relief bakingstep. In some embodiments, the chromate treatment increases thecorrosion protection for zinc/nickel plating and provides a goodadherent base for paint. Conventional chromating and other optionalpost-treatment steps are utilized as desired. The example below includesa chromating step.

Chromate is applied from a known process, basically a dip/immersionprocess, a non-electrodeposition process. The chromate is delivered fromvarious sources. Any chromate can be used and various chromate solutionscan be used, for example, chromic acid with sulfuric acid; a dichromateand sulfuric acid, or dichromic acid with hydrochloric acid.

Conventional pre and post treatment methods are described in U.S. Pat.No. 4,765,871, assigned to Boeing and incorporated by reference hereinin its entirety.

An exemplary process is given in FIG. 1. It shows a preferred sequenceof aqueous degreasing, abrasive blasting, acid activation,electroplating, and hydrogen embrittlement relief baking, along with anoptional chromate conversion coating. Conventional procedures can beused for the various non-electroplating steps, as indicated in FIG. 1 byreference to aerospace industry standards.

In an exemplary embodiment, aqueous degreasing is carried out accordingto the procedure and parameters prescribed in Boeing specifications BAC5408, BAC 5750, or BAC 5763, the full contents of which are incorporatedby reference.

Abrasive blasting is preferably performed after the degreasing. Anexemplary procedure is given in Boeing standard BAC 5748. In a preferredembodiment, abrasive blasting using a suitable grit (such as aluminumoxide) of 80 to 120 particle size (as defined in BAC 5748) is found tocontribute significantly to the low embrittlement/re-embrittlementproperties. Following grit blasting, acid activation of the substratejust prior to immersion into the plating bath tends to reduce oxideformation and results in strongly adhering plating deposits.

Alkaline zinc nickel plating is carried out using the bath compositionand plating parameters described herein. Table 1 summarizes variouspreferred alloy ranges, bath compositions, and operating parameters setforth above. Concentrations are given in ounces per gallon. One ounce(28.35 g) per gallon (3.79 liters) is equivalent to about 7.5 grams perliter (7.5 g/L). Temperatures are given in degrees Fahrenheit (° F.). Toconvert to ° C., subtract 32 and multiply the remainder by 5/9.

TABLE 1 Material/Condition Range Exemplary Alloy composition, as Zn 85to 95 plated (weight %) Ni 5 to 15 Zinc metal content 1.17 to 1.45oz/gal 1.4 oz/gal (bath) Nickel metal content 0.12 to 0.15 oz/gal 0.13oz/gal (bath) Zinc to Nickel weight 10:1 to 11:1 Ratio (no units) (bath)Nickel complexing agent Vary as needed (bath) Sodium Hydroxide 17.4 to21.2 oz/gal 18 oz/gal (bath) pH (bath) 12.0 to 13.5 12.5 Typical valuesSodium Carbonate (bath) Less than 8 oz/gal Agitation/FiltrationMechanical (2 to 3 volume (bath) turnovers per hour) 5 to 10 micronfilter Temperature (bath) 65° F. to 85° F. 70° F. to 80° F. Anoding 1)Main (primary): Nickel Caution: do not use slab, nickel plated nickelballs in titanium 2) Auxiliary (secondary): containers (poor Nickel,nickel plated, conductivity) platinized titanium Anode/Cathode RatioMinimum 2:1 Maximize Cathode Current Density 30 to 60 Amperes per 45Amperes per square foot square foot

If the Ni-complexing agent is omitted from the plating solution, but theother operational parameters of the present invention are maintainedwithin ranges defined below, a zinc/nickel coating exhibiting goodcorrosion resistance and low hydrogen embrittlement is still achieved.The nickel distribution in the coating is not, however, as good as whenthe nickel complexing agent is present in the plating solution. In orderto achieve this result, the plating bath composition is as per theExamples below. Brighteners are not included and preferably the onlyorganic compound in the bath is the nickel complexing agent. Grainrefiners can be included if they do not interfere with the deposition ofa suitably porous coating.

In summary, several factors were found to be useful for achieving thelow embrittlement and high corrosion resistance properties. Theseinclude:

-   -   Bath chemistry composition and concentration ranges    -   Plating parameters for temperature, current densities, and        anode-to-cathode ratios    -   Process sequence    -   Structure/morphology of the plate deposit        It should be noted that established low embrittlement plating        processes such as Cadmium-Titanium plating or Acid Zinc/Nickel        plating rely on additives to achieve the desired deposit        morphology. The process described herein does not use such        additives and relies solely on the control of process parameters        as described herein and exemplified below to produce the desired        structure. Additives such as are used with Cd—Ti or acid Zn/Ni        plating would potentially build up over time and would therefore        limit the bath life, unless removed automatically or through        incorporation into the plate deposit. For example, in        cadmium-titanium plating, where a titanium compound is        co-deposited, the excess titanium product is kept within a        filter system external to the actual plating bath. With        zinc/nickel plating baths, such methodology is technically not        feasible, and for that reason an additive-free bath chemistry,        which achieves low embrittlement deposits, turns out to be        advantageous.

The present invention is thus in various embodiments a moreenvironmentally acceptable plating process for high strength low alloysteels that does not introduce hydrogen embrittlement problems, or atleast minimizes such problems. The low hydrogen embrittlementzinc/nickel plating process of the invention is alkaline in nature andutilizes commercially available chemicals for plating bath preparationand replenishment. The deposited coating is primarily 85 to 95 percentzinc by weight and 5 to 15 percent nickel by weight, and will have adull appearance, over a wide range of current density, an effectiverange of 30 to 68 ampere-per-square-foot. The engineering properties ofthis coating are low embrittlement, good corrosion resistance, excellentpaint base and lubricity. This process can be used on steelsheat-treated at 220 ksi tensile strength or higher, replacing lowhydrogen embrittlement cadmium and low hydrogen embrittlementcadmium-titanium plating.

In preferred embodiments, the electroplating processes described hereinresult in a coating morphology that leads to both high corrosionresistance (indicating some resistance to allowing oxygen or waterthrough the coating to the alloy surface) and low hydrogen embrittlement(indicating that the coating provides a path for hydrogen to escape fromthe substrate). The morphology is thus described as dense enough toprevent oxygen or water from reaching the surface, but porous enough topermit hydrogen absorbed into the substrate to escape during a lowembrittlement bake subsequent to coating. Thus the morphology ischaracterized by a density that provides corrosion resistance and alsoby a porosity that provides low hydrogen embrittlement. This morphology,achieved here for the first time on a high strength low alloy steel, isalso illustrated in the Figures. In various Figures, the density of theanti-corrosion coating can be appreciated, and channels for escape ofhydrogen are clearly visible.

EXAMPLE I

An aqueous electroplating bath containing no brightener was preparedcontaining per gallon of solution, 1.17 to 1.45 ounces of zinc, 0.12 to0.15 ounces of nickel, 17.4 to 21.2 ounces of sodium-hydroxide and aminor amount of Ni-complexing agent. The pH of the bath was 12 to 13.5and was maintained at about room temperature of 70° F. to 80° F. Onlynickel was employed as an anode. Test panels were prepared with currentdensities of about 30 to 68 ASF, and more specifically, deposition wasconducted at 30 ASF, 36 ASF, 45 ASF and 68 ASF. At a current density ofabout 30 to 68 amperes per square foot, the zinc/nickel alloy depositwas well-coated on the high strength steel.

EXAMPLE II

An acceptable source of zinc metal is available from Atotech and soldunder the trade names Reflectalloy and Reflectalloy ZNA. A zinc-metalsolution having the designation ZNA ZS supplies zinc and caustic soda tothe bath; and the zinc compound has CAS no. 12179-14-5 designation andis present at up to about 10 wt % in the ZNA ZS and the sodium hydroxidehas CAS no. 1310-73-2 designation and is present in the ZNA ZS at up toabout 30 wt %. Product designation ZNA-92 Ni—C supplies nickel to thebath; and the nickel sulfate has CAS no. 7786-81-4 designation and ispresent in the ZNA-92 Ni—C at up to about 40-70 wt %. Productdesignation ZNA-C9300 carrier is used along with the 92 Ni—C tofacilitate delivery of nickel; and product ZNA-C9300 carrier is amixture of diethylene triamine, CAS No. 111-40-0 designation, up to 30wt % and triethanolamine, CAS no. 102-71-6 designation, up to 10 wt % inthe C9300 carrier. Product ZNA-C9400 carrier also adjusts the nickeldelivery, and C9400 carrier has characteristics similar to the C9300carrier. Further characteristics of each of the ZNA ZS, ZNA-92 Ni—C, theC9300 and the C9400 are included in the MSDS and Atotech productliterature for each, incorporated herein by reference as a teachingtool. Thus, it can be seen that the plating solution essentiallycomprises a source of zinc, a source of nickel, a source of caustic sodaand a source of complexing or chelating agents to facilitate thedeposition of appropriate amounts of nickel with respect to zinc.

The solution in accordance with this present example contains thefollowing quantities to provide 100 gallons of suitable electroplatingsolution using commercially available starting materials. Supply zincand caustic soda to the bath via 35 gallons of Reflectalloy ZNA ZS.Supply nickel to the bath using 0.75 gallons Reflectalloy ZNA-92 Ni—Cwhich is a nickel containing component that is used for the initialbath, and to replenish nickel in an operating bath; and it is used incooperation with the ZNA-C9300 which is supplied in the amount of 4.5gallons to adjust the amount and rate of nickel deposit and ReflectalloyZNA-C9400 in an amount of 5.4 gallons to also adjust the rate of nickeldeposit. In a typical protocol Rayon grade sodium hydroxide 50% inliquid form provides bath conductivity and assists in dissolution of thezinc. In a typical protocol, the steps are as follows: First fill theplating tank equipped with preferred chiller and agitator to 25 percentof its desired final volume. Second, while agitating, add the ZNA-C9300.Next, add the required amount of the Reflectalloy ZNA-92 Ni—C. Then therequired amount of Rayon grade sodium hydroxide is added followed by theaddition of the zinc solution. Next, stirring is continued while therequired amount of the Reflectalloy ZNA-C9400 carrier is added.Sufficient water to bring the solution to its final desired volume andcomposition is added. Next, the solution is stirred and the nominalcomposition is within the ranges as shown in Table 1 and as described inthis example with preferably the composition being near the mid-range ofthe values given in Table 1. Coating was conducted at 36 ASF, 45 ASF and68 ASF.

The high strength steel specimens were tested by static tensile loadingat 75 percent of established notch ultimate tensile strength (oftenreferred to as the dry test). The specimens were loaded continuously forat least 200 hours. The specimens withstood the loading for more than200 hours and exhibit satisfactory low hydrogen embrittlementcharacteristics. Best results occurred at 45 ASF, giving a good coatingdistribution, good resistance to corrosion and good resistance tohydrogen embrittlement. Specimens at 36 ASF and 68 ASF also showed goodresults in all categories, but the specimen at 68 ASF showed slightcorrosion.

EXAMPLE 3

Using the process flow and plating parameters of the invention asoutlined above, a zinc/nickel plated structure with low embrittlementproperties is obtained while maintaining corrosion performance. WhenZn/Ni baths containing conventional brighteners are used, even with thesame process flow and plating parameters, a dense zinc/nickel platingstructure is formed where hydrogen bake-out is retarded. Theembrittlement performance can be correlated to the morphology of thecorresponding plated, as demonstrated in FIGS. 2 a and 2 b.

The following information is applicable for the LHE Zn/Ni plating asillustrated in FIG. 2 a. The substrate is AISI 4340M steel plate. Thesurface is degreased, blasted, acid activated and plated at 45 ASF forapproximately 30 minutes to give a coating of approximately 0.8 mil(0.0008 inch):

1) LHE Zn—Ni Chemistry & Parameters g/L/7.488 = oz/gal Concentration ofTank Control range Item Contain Provided by g/L oz/gal g/L oz/gal 1 Znmetal ZNA ZS 10.77 1.44  8.7-10.8 1.2-1.45 2 NaOH 135.00 18.03129.8-158.2 17.4-21.2  3 Ni metal ZNA-92 Ni—C 1.06 0.14  0.9-1.120.12-0.145 4 Zn:Ni ratio 10.20 10:1-11:1 5 Carrier (Ni ZNA-C9300 Carrier~4.5% by volume ~4.5% by volume complexer) 6 Carrier (Grain ZNA-C9400Carrier ~5.4% by volume ~5.4% by volume refiner) 7 Brightener ZNA-9500None None 8 Brightener ZNA-9700 None None 9 Sodium By product 40.00 5.34<59.8 <8 Bicarbonate 10 Temperature Heater/chiller 70 F. 70-80 F. 11Current density Rectifier 45 ASF 30-65 ASF

The following information is applicable for the bright Zn/Ni plating asillustrated in FIG. 2 b. The substrate is AISI 4130 annealed steelsheet. The substrate surface is degreased, blasted, alkaline cleaned,acid activated and plated at 25 ASF for approximately 30 minutes to givea coating of 0.5 mil (0.0005 inch):

2) Bright Zn—Ni Chemistry & Parameters g/L/7.488 = oz/gal Concentrationof Tank Control range Item Contain Provided by g/L oz/gal g/L oz/gal 1Zn metal ZNA ZS 11.00 1.47 8.7-12.5 1.2-1.7 2 NaOH 140.00 18.70129.8-158   17.4-21.2 3 Ni metal ZNA-92 Ni—C 1.10 0.15 0.9-1.130.12-0.17 4 Zn:Ni ratio 10.00 8:1-11:1 5 Carrier (Ni ZNA-C9300 Carrier~4.5% by volume ~4.5% by volume complexer) 6 Carrier (Grain ZNA-C9400Carrier ~5.4% by volume ~5.4% by volume refiner) 7 Brightener ZNA-9500~0.033% by volume  ~0.033% by volume  8 Brightener ZNA-9700 ~0.3% byvolume ~0.3% by volume 9 Sodium By product 50.00 6.68 <59.8 <8Bicarbonate 10 Temperature Heater/chiller 70 F. 68-85 F. 11 Currentdensity Rectifier 20 ASF 15-30 ASF

FIGS. 2 a and 2 b show the top surface of the plated substrate, whileFIGS. 3 a and 3 b are fracture cross section views of the zinc/nickelplate deposit of FIGS. 2 a and 2 b respectively. FIGS. 4 through 5 areadded for comparison purpose to show the conventional structures of LowHydrogen Embrittlement Cadmium-Titanium Alloy plate and the bake-outretarded structure of bright cadmium plate. Those are the processescurrently in use throughout the industry.

FIG. 2 a shows a low embrittlement alkaline Zn—Ni plate deposit of theinvention at 500×, top surface. The coating has a morphologycharacterized by a microporosity that permits entrapped hydrogen tomigrate out of the plate.

FIG. 2 b shows a bright Zn—Ni plate deposit (hydrogen bake-outinhibited) at 500×, top surface. This coating was produced with anelectrodeposition bath that contained conventional brighteners. Thisdense deposit, also known as “bright” plating contains almost nomicropores and will not permit hydrogen to migrate through.

FIG. 3 a shows the plate of FIG. 2 a at a plate bend fracture at 500×.Notice the nodular structure which permits hydrogen bake out.

FIG. 3 b shows the plate of FIG. 2 b in a plate bend fracture at 2000×.Notice the dense fracture layer (light band in center) which retardshydrogen bake out and leads to substrate embrittlement.

For comparison, FIG. 4 a shows a top view of standard low embrittlementCadmium-Titanium plate at 500×, showing a coating on a HSLA steel.Notice the typical crystalline porous microstructure of the deposit,which permits hydrogen bake-out. This porous morphology is successfullyformed in FIG. 2 a using an alkaline Zn/Ni bath with no organics (otherthan the Ni complexing agent).

In Contrast, FIG. 4 b shows a standard dense bright cadmium deposit at500×. The deposit does not have low embrittlement properties, because ofits dense structure. Note the similarity to the morphology in FIG. 2 b,prepared with a conventional Zn/Ni bath.

FIGS. 5 a and 5 b show the platings of 4 a and 4 b, respectively, in aplate bend fracture. Note the similarity of the morphology of 5 a to 3a, and 5 b to 3 b. In FIG. 6 a, the substrate is aluminum-nickel bronzesteel rod. The substrate surface is degreased, blasted, acid activatedand plated at 25 ASF for 20 minutes to give a coating of 0.2 mil. InFIG. 6 b, the substrate is 4340M steel and prepared and plated asfollows: degreased, blasted, acid activated, and plated at 45 ASF for 35minutes to give a coating of 0.8 mil. The coating is prepared in amanner very similar to that of FIG. 2 a.

The chemistry and parameters used to obtain coating as illustrated inFIG. 6 is shown below (similar to what shown above. Because thesespecimens were plated from the same bath within a day of each other):

1) LHE Zn—Ni Chemistry & Parameters g/L/7.488 = oz/gal Concentration ofTank Control range Item Contain Provided by g/L oz/gal g/L oz/gal 1 Znmetal ZNA ZS 10.77 1.44 8.7-10.8 1.2-1.45 2 NaOH 135.00 18.03129.8-158.2  17.4-21.2  3 Ni metal ZNA-92 Ni—C 1.06 0.14 0.9-1.120.12-0.145 4 Zn:Ni ratio 10.20 10:1-11:1 5 Carrier (Ni ZNA-C9300 Carrier~4.5% by volume ~4.5% by volume complexer) 6 Carrier (Grain ZNA-C9400Carrier ~5.4% by volume ~5.4% by volume refiner) 7 Brightener ZNA-C9500None None 8 Brightener ZNA-C9700 None None 9 Sodium By product 40.005.34 <59.8 <8 Bicarbonate 10 Temperature Heater/chiller 70 F. 70-80 F.11 Current density Rectifier 45 ASF 30-65 ASF

In the Examples, brighteners ZNA-C9500 and ZNA-C9700 are commercialproducts of Atotech.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A method for producing a plated low alloy strength steel havingsuperior corrosion and low hydrogen embrittlement properties, the methodcomprising: electroplating a zinc/nickel coating on a low alloy, highstrength steel from an alkaline plating bath, wherein the bath compriseszinc and nickel at a level to produce a plated coating containing 85-95%by weight zinc and 5-15% by weight nickel, wherein the bath comprises aneffective amount of a nickel compounding agent; wherein the bathcomprises less than 100 ppm of any organic material other than thecomplexing agent; and wherein the coating has a dense, porous morphologythat provides corrosive resistance on account of its density andresistance to hydrogen embrittlement on account of its porosity.
 2. Amethod according to claim 1, wherein the bath contains less than 10 ppmof the organic material.
 3. A method according to claim 1, wherein thebath contains less than 1 ppm of the organic material.
 4. A methodaccording to claim 1, wherein the pH of the bath is 12-13.5, and theelectroplating is carried out with a cathode current density of 30-60amperes per square foot.
 5. A method according to claim 4, wherein thetemperature of the bath during electroplating is 70° F.-80° F.
 6. Amethod according to claim 1, wherein the bath has a pH of 12-13.5 andcomprises less than 50 ppm of any organic material other than thecomplexing agent; 1.17-1.45 ounces per gallon of Zn metal; 0.12-0.15ounces per gallon of Ni metal, wherein the rate ratio of zinc to nickelin the bath is from 10:1 to 11:1; wherein the electroplating is carriedout at a cathode current density of 30-60 amperes per square foot and ata bath temperature of 70-80° F.
 7. A method of making an electroplatedzinc nickel coated high strength low alloy steel part, comprisingaqueous degreasing a low alloy high strength steel part; grit blastingthe degreased and cleaned part; acid activating the grit blasted steelpart; electroplating the acid activated steel part in an alkaline Zn/Niplating bath; and hydrogen embrittlement relief baking the electroplatedsteel part; wherein the plating bath comprises zinc and nickel at alevel to produce a plated coating containing 85-95% by weight zinc and5-15% by weight nickel, the bath comprises an effective amount of anickel compounding agent; and the bath comprises less than 100 ppm ofany organic material other than the complexing agent; and the coatinghas a dense, porous morphology that provides corrosive resistance onaccount of its density and resistance to hydrogen embrittlement onaccount of its porosity.
 8. A method according to claim 7, furthercomprising applying a chromate conversion coat onto the relief bakedsteel part.
 9. A method according to claim 7, wherein the bath comprisesless than 10 ppm of the organic material.
 10. A method according toclaim 7, wherein the bath comprises less than 1 ppm of the organicmaterial.
 11. A method according to claim 7, wherein the bath has a pHof 12-13.5 and comprises less than 50 ppm of any organic material otherthan the complexing agent; 1.17-1.45 ounces per gallon of Zn metal and0.12-0.15 ounces per gallon of Ni metal, wherein the rate ratio of zincto nickel in the bath is from 10:1 to 11:1; wherein the electroplatingis carried out at a cathode current density of 30-60 amperes per squarefoot and at a bath temperature of 70° F.-80° F.
 12. A method accordingto claim 7, wherein the bath comprises zinc oxide and nickel sulfate.13. A low alloy steel part made by a process according to claim
 7. 14. Azinc nickel plated article comprising a substrate and a zinc nickelalloy plating on the substrate, wherein the substrate is a high strengthlow alloy steel, wherein the plating is 85-95% by weight Zn and 5-15% byweight Ni and has a morphology that is dense and porous, and wherein thearticle meets corrosion specification requirements of ASTM B-17 and thelow embrittlement specification requirements of ASTM F-S 19, Type 1a.2.15. A plated article according to claim 14, wherein the substrate is4340 steel alloy.
 16. A plated article according to claim 14, whereinthe substrate is 4340M steel alloy.
 17. A plated article according toclaim 14, wherein the article is a part of a commercial aircraftcomponent.
 18. A plated article according to claim 14, wherein theplating is characterized by a density sufficient to provide the observedcorrosion resistance, and by a porosity sufficient to provide theembrittlement resistance.
 19. A plated article according to claim 14,made by a process comprising electroplating a zinc nickel coating on alow alloy high strength steel from an alkaline plating bath, wherein thebath comprises zinc and nickel at a level to produce a plated coatingcontaining 85-95% by weight zinc and 5-15% by weight nickel, wherein thebath comprises an effective amount of a nickel compounding agent;wherein the bath comprises less than 100 ppm of any organic materialother than the complexing agent; and wherein the coating has a dense,porous morphology that provides corrosive resistance on account of itsdensity and resistance to hydrogen embrittlement on account of itsporosity.
 20. A plated article according to claim 19, made by a processcomprising aqueous degreasing and alkaline cleaning a low alloy highstrength steel part; grit blasting the degreased and cleaned part; acidactivating the grit blasted steel part; electroplating the acidactivated steel part in an alkaline Zn/Ni plating bath; and hydrogenembrittlement relief baling the electroplated steel part; wherein thebath comprises zinc and nickel at a level to produce a plated coatingcontaining 85-95% by weight zinc and 5-15% by weight nickel, wherein thebath comprises an effective amount of a nickel compounding agent;wherein the bath comprises less than 100 ppm of any organic materialother than the complexing agent; and wherein the coating has a dense,porous morphology that provides corrosive resistance on account of itsdensity and resistance to hydrogen embrittlement on account of itsporosity.